1. Field of the Invention
The present invention relates to a video signal converter and a method for converting a video signal composed of luminance signals of each primary color (RGB) which may have a signal value within an extended range partially including a prescribed output range into a video signal composed of luminance signals of each primary color (RGB) which may have a value within the output range, as well as to a video display device comprising the video signal converter. In addition, the extended range is what the output range is extended, and is wider than the output range.
2. Description of the Related Art
In a video display device such as a television receiver, a video display member such as a liquid crystal display displays a video based on a video signal (hereinafter referred to as “output side video signal”) composed of luminance signals of each primary color (RGB). Generally, in regard to a luminance signal of primary colors (RGB) in the output side video signals, its signal value, in short, a value of a luminance signal of each Red, Green, and Blue is normalized within an output range (for example, such as 0 to 255 or 0 to 1) from a predetermined lower limit value (hereinafter referred to as “output lower limit value”) to an upper limit value greater than the lower limit value (hereinafter referred to as “output upper limit value”). Here, a color, which is expressed by a combination of values of luminance signals of primary colors (RGB) within the output range, is the color in a color gamut displayable by a video display member. In addition, the color gamut displayable, that is, reproducible by the video display member is referred to as a color reproduction region or a color reproduction range of the video display member. Additionally, in the present Description, the words “displayable” and “reproducible” mean the same.
When a color gamut possible to be expressed by an input side video signal agrees with a color gamut possible to be reproduced by a video display member, the input side video signal as it is may be employed as the output side video signal. This allows a video of a color corresponding to the input side video signal to be displayed on the video display member. Additionally, the input side video signal is a video signal to be input into a video display device (hereinafter referred to as “input video signal”), or a video signal obtained by subjecting the input video signal to a known color gamut converting processing.
On the other hand, like the case where a color gamut possible to be expressed by an input side video signal is wider than a color gamut of the video display member, when a color gamut possible to be expressed by the input side video signal is outside of a color gamut possible to be reproduced by a video display member, the signal value of the input side video signal may obtain a value outside of the output range. The input side video signal as it is cannot therefore be employed as the output side video signal.
For example, there is a case where a luminance value range of three primary colors in the video signal corresponding to the color gamut of a video display member, in short, the output range is 0 to 1, whereas a signal value of the input side video signal may be negative or more than 1. In such a case, it is necessary to convert the input side video signal into the output side video signal composed of signal values within the output range. As a state requiring such conversion, for example, the input side video signal is a video signal complying to the standard IEC 61966-2-4 (commonly known as “xvYCC standard”) and IEC 61966-2-1, whereas the output side video signal may be a video signal complying to the standard ITU-R BT.709.
Here, as a signal converting method when a signal value of the input side video signal is outside of the output range, the output side video signal may be converted by clipping a signal value of the input side video signal in the output range. The clipping processing is an easiest signal conversion method and also referred to as “limiter processing”.
In addition, the Patent Document: Japanese Unexamined Patent Publication No. H09-98298 discloses an art in that, when a color gamut of the output system is narrower than that of the input system, the color gamut of the input system is region-divided on a two-dimensional plane of lightness and saturation, and then the compression of the color gamut, in short, the compression of the signal value is conducted, while setting the color phase constant in each divided region. Additionally, the color gamut of the output system corresponds to a color gamut of a video display member, while the color gamut of the input system corresponds to a color gamut of the input side video signal.
However, to convert the input side video signal into the output side video signal by the clipping processing has a problem as below. That is, since all signal values of the input side video signal that exceed the output range are replaced with the output lower limit value or the output upper limit value, the color continuity in the input side video signal is seriously impaired, in short, a gradation failure occurs.
On the other hand, as can be seen from the Patent Document, to convert the input side video signal into the output side video signal by the color gamut compression has a problem as below. That is, even when the input side video signal expresses a color within the color gamut of the video display member, in short, even when a signal value of the input side video signal is within the output range, the video signal might be converted into a video signal of a different color, though the color continuity may be ensured. This causes a problem in that an original color the input video signal ought to express cannot be reproduced (displayed) accurately. Moreover, in the processing shown in the Patent Document, a video signal is processed as data on the two-dimensional plane of lightness and saturation. The processing shown in the Patent Document for conducting the color gamut compression as keeping the color phase constant therefore becomes an arithmetic processing using a trigonometric function, thereby involving a high arithmetic load.
Consequently, the present invention has been completed on the basis of the above circumstances, with an object of providing a video signal converter, a video signal converting method, and a video display device which are possible to accurately reproduce (express) an original color a given video signal ought to express, when a color gamut of the given video signal corresponding to the input side video signal does not agree with a color gamut displayable by a video display member, and thus the video signal is converted into a video signal expressing a color within the color gamut of the video display member, and when the given video signal expresses a color within the color gamut of a video display member, and furthermore, possible to ensure the color continuity, in short, possible to prevent a gradation failure when the given video signal expresses a color in the region outside of the color gamut of the video display member, while at the same time, possible to perform a signal conversion processing at a low arithmetic load.